Annual Broadleaf Weed Control
Annual broadleaf weeds are more competitive with soybeans than are annual grasses. Annual broadleaf weeds also vary greatly in their potential to reduce soybean yield. You should strive to obtain excellent control of the more competitive species such as cocklebur, lambsquarter, jimsonweed, and smartweed. A fairly high population of the less competitive species such as prickly sida and tropic croton may not seriously reduce soybean yields.
Annual broadleaf weeds can be controlled with preplant-incorporated, preemergence, and postemergence herbicides. The best application method to use depends upon the weed species present and compatibility with your overall production system. This section discusses the various herbicide options.
Soil-Applied Herbicides for Broadleaf Weeds
Grass control herbicides. Soil-applied grass herbicides also control certain broadleaf weeds. Prowl, Sonalan, Treflan, and Vernam control annual grasses plus certain small-seeded broadleaf weeds such as pigweed and lambsquarter (Table 1). If the predominant weed problem is annual grasses plus pigweed, lambsquarter, or both, one of these preplant-incorporated herbicides plus cultivation may be adequate.
Command controls prickly sida, tropic croton, jimsonweed, lambsquarter, smartweed, spurred anoda, and velvetleaf. Note that Command does not control pigweed. Dual or Lasso will control pigweed.
Lexone, Sencor, Salute, and Turbo. These products containing metribuzin normally control many annual broadleaf weeds. Notable exceptions include cocklebur and morning glory. Lexone, Sencor, and Turbo can be preplant incorporated or applied preemergence. Salute should always be incorporated.
Salute is a packaged mixture containing trifluralin plus metribuzin (the active ingredients in Treflan and Sencor, respectively). Turbo is a packaged mixture containing metolachlor plus metribuzin (the active ingredients in Dual and Sencor, respectively).
Soybeans have a narrow margin of tolerance for metribuzin. Therefore, be extremely careful when selecting the application rate of a product that contains metribuzin. This can be a problem in fields with varying soil types. Miles Inc., makers of Salute, Sencor, and Turbo, offer a free soil testing program called SURE. The SURE program recommends a specific rate of Salute, Sencor, or Turbo based upon soil texture, organic matter content, and application method (preplant incorporated or preemergence). SURE has proven to be very helpful in selecting the proper metribuzin rate to avoid crop injury.
There are several restrictions on the use of metribuzin. Specific restrictions vary depending upon application methods and tank mixes, but in general this herbicide should not be used on sands or loamy sands with less than 1 percent organic matter or any soil with less than 1/2 percent organic matter. Certain varieties of soybeans, such as Asgrow 6520 and Coker 156, are very susceptible to metribuzin injury. Before applying a product containing metribuzin to a new variety, check with your seed dealer to determine the variety’s sensitivity to metribuzin. Increased soybean injury will also be observed when metribuzin is used in conjunction with soil-applied organophosphate insecticide/nematicides; see the product labels for details.
Canopy. Canopy is a packaged mixture containing 6 parts metribuzin to 1 part chlorimuron (Classic). Because it contains a high percentage of metribuzin, the use restrictions discussed previously for Lexone, Salute, Sencor, and Turbo also apply to Canopy. Canopy can be applied in various preplant-incorporated or preemergence tank mixes.
In contrast to metribuzin alone, Canopy provides better control of large-seeded broadleaf weeds such as cocklebur and morningglory. Canopy normally controls most annual broadleaf weeds. In fields heavily infested with cocklebur or morningglory, cultivation or a postemergence herbicide may also be needed.
Some rotational restrictions apply when using Canopy. However, there have been no documented cases of Canopy carryover to agronomic crops in North Carolina. Carefully follow label directions for sprayer cleanout after using Canopy.
Lorox. This preemergence herbicide controls common ragweed, pigweed, and lambsquarter but is inadequate for control of most other broadleaf weeds.
Lorox should not be used on sand, loamy sand, or gravelly soils or on any soil with less than 1/2 percent organic matter. Lorox is somewhat safer on light- textured soils than is metribuzin. However, injury may occur if excessive rates are applied or if heavy rainfall is received shortly after planting. Do not use on soils with more than 3 percent organic matter.
Scepter, Squadron, and Tri-Scept. Scepter can be applied in various preplant-incorporated or preemergence tank mixes. The most consistent and broadest spectrum control is obtained when Scepter is tank-mixed with Prowl or Treflan and incorporated.
More rainfall is required to activate a preemergence application of Scepter than is required for most preemergence herbicides. For good control, 3/4 to 1 inch of rainfall is needed within 7 to 10 days after application. For this reason, incorporated applications perform more consistently.
Soil-applied Scepter controls most broadleaf weeds. Major exceptions are tropic croton, hemp sesbania, spurred anoda, and volunteer cowpea. Morningglory control is variable but usually adequate, especially when followed by cultivation.
Soybean tolerance of Scepter is normally good. However, slight to severe crop stunting has occasionally been noted. Heavy rainfall and waterlogged soils shortly after planting appear to enhance the problem. To reduce the potential for crop stunting, avoid deep incorporation and shallow planting, especially on sandy soils.
Squadron is a packaged mixture containing pendimethalin (Prowl) and imazaquin (Scepter); Tri-Scept is a packaged mixture containing trifluralin (the active ingredient in Treflan) and imazaquin. Weed control and crop tolerance with Squadron or Tri-Scept are similar to that with a tank mix of Prowl or Treflan plus Scepter.
Squadron can be preplant incorporated or applied preemergence. Incorporated applications are generally preferred, especially in fields with moderate to heavy infestations of annual grasses. Tri-Scept should always be incorporated.
Note the 11-month rotational restriction for corn. Although not widespread, some carryover to corn has been noted in North Carolina. Carryover has generally been confined to very sandy soils where Scepter, Squadron, or Tri- Scept was incorporated the previous year, where rainfall the previous growing season was below normal, and where soil pH was well below optimum levels. In light of the potential for carryover to corn, the 11-month restriction should be adhered to closely. Do not plant cotton or vegetables the year after applying Scepter, Squadron, or Tri-Scept.
Broadstrike + Dual and Broadstrike + Treflan. These packaged mixtures contain flumetsulam plus Dual or Treflan. Broadstrike + Dual can be preplant incorporated or applied preemergence; Broadstrike + Treflan can be preplant incorporated only.
In addition to the weeds normally controlled by Dual or Treflan, Broadstrike + Dual or Broadstrike + Treflan control a number of broadleaf weeds. Notable exceptions include hemp sesbania and giant ragweed. Broadstrike + Dual and Broadstrike + Treflan may not adequately control moderate to heavy infestations of cocklebur, common ragweed, and morningglory. Crop tolerance of Broadstrike + Dual and Broadstrike + Treflan appears to be good. There are significant rotational restrictions for both herbicides.
Postemergence Herbicides for Broadleaf Weeds
All common annual broadleaf weeds can be controlled with postemergence herbicides. However, weed species vary in their response to the different herbicides, and proper weed identification and herbicide selection are essential for satisfactory results.
In addition to proper herbicide selection, timeliness of application and proper application methods are essential. The most common cause of poor results with postemergence herbicides is application when the weeds have grown beyond the optimum size for treatment. As a general rule, postemergence broadleaf herbicides should be applied when weeds are in the two- to four-leaf stage or 2 to 3 inches tall. This normally occurs 2 to 3 weeks after planting.
Weather conditions are also important to the success of postemergence herbicides. For best results, weeds should be actively growing and not under moisture stress. When weeds are under moisture stress, they are tough and less herbicide is absorbed into the plant, resulting in less control.
Many of the postemergence broadleaf herbicides act through contact, meaning that there is little to no translocation or movement of the herbicide within the plant. Consequently, thorough spray coverage is necessary for good results. See Table 9 for recommended application pressures and volumes. Postemergence herbicides should be applied using either flat fan nozzles or hollow cone nozzles. Flood nozzles give poor spray coverage and should never be used to apply postemergence herbicides. Most postemergence herbicide labels also specify not to use CDA (controlled droplet applicator) sprayers. See specific labels for application directions.
Spray adjuvants are normally used with postemergence herbicides.
Basagran. Basagran gives excellent control of cocklebur, jimsonweed, and smartweed, and good control of prickly sida, spurred anoda, velvetleaf, and giant ragweed. Although control may be inconsistent, Basagran plus crop oil concentrate usually controls small common ragweed and lambsquarter. Crop tolerance is excellent.
Blazer. Blazer controls most broadleaf weeds. Exceptions include prickly sida, sicklepod, spurred anoda, velvetleaf, and volunteer cowpea.
Blazer normally causes some soybean leaf crinkling and leaf bronzing or leaf burn. Addition of crop oil concentrate or higher rates of surfactant increase the amount of leaf burn. This injury is temporary and the soybeans recover quickly and grow normally.
Basagran Plus Blazer, Storm. A tank mix of Basagran plus Blazer is a very effective, broad-spectrum treatment. Application of 1 pint of Basagran plus 1 pint of Blazer plus an adjuvant has proven to be acceptable in most situations. However, the rate of Basagran and Blazer can be increased to 2 pints and 1.5 pints per acre, respectively, if needed. The best rate of each herbicide to use in the tank mix depends upon the weed species and weed size; see the product labels for details.
Storm is a packaged mixture containing the active ingredients found in both Basagran and Blazer. Applying 1.5 pints of Storm is equivalent to applying 1 pint of Basagran plus 1 pint of Blazer. One pint per acre of crop oil concentrate is always recommended with Storm.
Classic. Classic will control many broadleaf weeds. Major exceptions are lambsquarter, prickly sida, and tropic croton. Control of spurred anoda and velvetleaf also may not be adequate. Classic may be tank-mixed with several other herbicides to expand the spectrum of control.
Soybean tolerance of Classic is normally good, but some leaf distortion and yellowing and minor crop stunting may be noted, especially when applied to soybeans under stress conditions. The soybeans usually recover and grow normally.
See Table 14 for rotational restrictions following Classic application. No documented cases of Classic carryover to agronomic crops have been observed in North Carolina. See the label for specific directions on sprayer cleanout.
Cobra. Cobra controls most annual broadleaf weeds. Major exceptions are lambsquarter, sicklepod, smartweed, and spurred anoda. Morningglory control often is inconsistent. Cobra applied at the full labeled rate (12.5 ounces per acre) usually causes moderate to severe soybean leaf burn. Although the soybeans usually recover and grow normally, the injury often is unacceptable to growers. For this reason, Cobra at the full labeled rate is generally not recommended. Several tank mixes containing Cobra at low rates (6 ounces per acre) are registered.
Pinnacle. Pinnacle controls lambsquarter, pigweed, smartweed, and velvetleaf. Because the spectrum of control is limited, Pinnacle is normally tank-mixed with another broadleaf herbicide to expand the range of species controlled. Carefully follow label directions for herbicide and adjuvant rates when applying tank mixes containing Pinnacle.
Reflex. Reflex controls most common broadleaf weeds. Major exceptions are prickly sida, sicklepod, spurred anoda, and velvetleaf. Unless treated when smaller than 1/2 inch, lambsquarter will not be controlled. Soybean tolerance of Reflex is normally good. Although the Reflex label mentions rotational restrictions, carryover to agronomic crops has not been observed in North Carolina.
Typhoon is a packaged mixture containing fomesafen and fluazifop (the active ingredients in Reflex and Fusilade, respectively). Typhoon at the recommended rate of 3.2 pints per acre is equivalent to 1.5 pints of Reflex and 12 ounces of Fusilade DX per acre.
Scepter. Scepter applied postemergence controls cocklebur and pigweed excellently. Control of other broadleaf weeds will be inadequate. See the section on sicklepod control.
Soybean tolerance of Scepter applied postemergence is excellent. See the most recent label for rotational restrictions.
Scepter O.T. Scepter O.T. is a packaged mixture of imazaquin and acifluorfen (the active ingredients in Scepter and Blazer, respectively). Applying 1 pint of Scepter O.T. per acre is equivalent to applying 1/3 pint of Scepter plus 1 pint of Blazer. Scepter O.T. gives excellent control of cocklebur and pigweed and will control other weeds normally controlled by 1 pint of Blazer.
Pursuit. Applied postemergence, Pursuit controls cocklebur, pigweed, jimsonweed, and smartweed. Morningglory control is usually acceptable but can be inconsistent.
Pursuit kills weeds very slowly, and it may take as long as 3 weeks to kill morningglory. In some cases, the weeds do not die completely but rather stop growing and the soybean canopy fills in above them. In addition to controlling certain broadleaf weeds, a postemergence application of Pursuit usually gives adequate control of broadleaf signalgrass, foxtails, seedling johnsongrass, and shattercane, and it sometimes gives adequate control of rhizome johnsongrass. Pursuit does not control common ragweed, lambsquarter, prickly sida, sicklepod, and certain other weeds.
Soybean tolerance of Pursuit is good. See Table 14 for rotational restrictions. Do not plant cotton or most vegetable crops the year following a Pursuit application.
Postemergence-Directed Herbicides for Broadleaf Weeds
Although not widely used in North Carolina, postemergence-directed herbicides are effective, economical tools for controlling late-emerging weeds or weeds that have escaped at-planting or early-season postemergence overtop herbicides. Directed applications are also very effective as a component of a sicklepod management program.
Herbicides and tank mixes labeled for postemergence-directed application to soybeans include Gramoxone Extra, Lexone, Lorox, 2,4-DB (Butyrac, Butoxone, and others), Lorox plus 2,4-DB, Sencor, and Sencor plus 2,4-DB. A surfactant should be added according to label directions to all of these herbicides and tank mixes except 2,4-DB applied alone.
For postemergence-directed treatments to be effective, the weeds must be shorter than the crop. The soybeans should be at least 8 inches tall (12 inches for Lexone) and the weeds should be 3 to 4 inches tall or less, depending upon the herbicide and application rate selected. Precision application is necessary to avoid unacceptable crop injury. The spray solution should contact no more than the lower 1/4 to 1/3 (or about 3 inches) of the soybean plant. Nozzles should be mounted in a manner that allows good control of the spraying height. Do not use booms with drop nozzles. Mount the nozzles on a cultivator frame or use a sliding shoe spray rig. Shielded sprayers are particularly effective. Use regular flat fan nozzles (or offset flat fan nozzles for banding) and a maximum pressure of 25 psi. For additional details, see Extension Service publication AG-259, Application of Directed Postemergence Herbicide in Cotton and Soybeans.
Depending on the herbicide chosen, directed sprays will control a broad spectrum of annual grasses and broadleaf weeds. 2,4-DB is effective on cocklebur and morningglory; treatment is suggested when weeds are 3 inches tall or less, although much larger weeds can be killed if spray coverage is good on the lower portion of the weeds. Gramoxone Extra controls pigweed and annual grasses up to 3 inches tall. Lexone or Sencor controls most species of broadleaf weeds up to 3 inches tall and annual grasses up to 1 inch tall. Lorox is effective on most annual broadleaf weeds and grasses up to 4 inches tall. A directed application of Lorox plus 2,4-DB has been very effective on a wide range of weed species in on-farm tests. For optimum yields, weeds must be controlled for the first 4 to 5 weeks after soybean planting. Weeds that emerge later have little direct effect on yield but may interfere with harvesting and add foreign matter to the harvested product.
Although you should strive for good early-season weed control, situations arise where a salvage treatment is necessary. Remember that salvage treatments are not a substitute for good early-season control.
2,4-DB (Butyrac, Butoxone, and others) can be applied over the top of soybeans at rates up to 1 pint per acre from 7 to 10 days before bloom up to mid-bloom. Application before or after this time period may result in severe crop injury. Application to soybeans under drought stress will also increase injury and reduce the chances of the crop recovering from the injury. This is a risky treatment and should be considered only as a salvage operation to remove cocklebur and morningglory when harvesting would otherwise be impossible.